Collisional nature of the magnetic field quenching of the acetylene

Laser fluorescence excitation from the V2Kn(n=0–2) levels of acetylene was measured as a function of acetylene pressure in the absence and in the presence of an external magnetic field. A magnetic field (H) accelerates the rate of intersystem crossing, directly affecting the dynamics of the à 1Au state. Studies carried out in the collisionless environment of a supersonic free-jet indicate that (1) the integrated fluorescence intensity remains the same upon applying H, (2) the fluorescence decay times increase with H, (3) the fluorescence decay amplitudes decrease with H. Therefore, molecules transferred to the triplet manifold can return to the singlet manifold and emit. In contrast, faster fluorescence decays are measured when the excited molecules are allowed to undergo collisions in the presence of H. The fluorescence is quenched because the system spends more time in the triplet manifold, which increases the chance of collisional deactivation. A discussion is presented in the context of the acetylene level structure, non-radiative processes, level coupling induced by H, and collisional relaxation.